I want to build a stereo headset valve amplifier

[Don_Zalmrol]

Hi Rob,

Then I will leave the resistors as they are.
I've redrawn the power circuit with an artificial center tap.

As from what I can understand from other online sources, it's designed to cancel out the 120 cycles hum (2x 60Hz), although here in Europe that would be 100 cycles hum (2x 50Hz). Correct me if I'm wrong.

I've drawn 2x 100 Ohms in series that are connected to the chassis ground.

[glowinganode]

Laurens, the junction of the two 100 ohm should go to ground. This is the only point of the heater wiring that should connect to ground.
For best effect, the two heater wires should be tightly twisted together, with no loops formed around the valve base.
Like you say, this helps to reduce hum pick up, although any hum would be at line frequency (50Hz).
It is only the rectifier that produces hum at twice line frequency (100Hz).
By the way, the mains fuse ought to go after the on-off switch.
Keep going, you're getting closer all the time.
Rob.

[Don_Zalmrol]

Hi Rob,

Take 4 (see the attachment).

The switch and fuse was a small error on my part, looked over it…

I will use 2x 2 tightly twisted cables for the 2x 12AU7 valves and the 2x 6CG7 valves.

I do however need to make a little loop (bridge) between the 12AU7 valves for powering pins 4 & 5 heater supply.

And also tap from the first valves to the others (i.e. from 1 12AU7 valve to the other and from 1 6CG7 valve to the other)

[glowinganode]

Hi Laurens,
as the saying goes, a picture paints a thousand words.
See attached photo's, excerpt taken from Amateur Radio Techniques, Pat Hawker.
Rob.

[Don_Zalmrol]

Thx, Chris you and this site is a goldmine

I've revised the circuit design thanks to your drawing

[Don_Zalmrol]

If everything is in order with the circuits I will place the order today.

Thank you all for your help!

[Don_Zalmrol]

I've ordered the first part from JR with the valves, transformer, casing, …

Now I'm ordering the other components from Distrilec Belgium.
I've came across a mains filter IEC socket. Should I order this one something similar?

Or without a filter?

[glowinganode]

Hi Laurens, the link just took me to the home page, but yes get one with a filter.
Rob.

[Don_Zalmrol]

Alrighty, thx.

Cheers,

Laurens

[Don_Zalmrol]

Just a small question, my order is almost complete and I just need to order the signal (coupling) caps.

Which ones should I order?

I need:
2x 1uF 200V
2x 10uF 200V

Will these suffice?

10uF 250VDC (PHE426HR8100JR06L2) and 1uF 250VDC (PHE450HD7100JR06L2)

If you are directed to the homepage of Distrilec, please see the attachments for the data sheets of the caps in question.

[Graham-R]

Just as a bit of an aside, has anyone else built the Elektor OTL headphone amp mentioned in some previous posts ?

I am really pleased with the sound of mine.

Graham

[Don_Zalmrol]

No, but I do find it an interesting project.
Did you used the PCB's or just point to point?

[Graham-R]

Originally Posted by Don_Zalmrol

No, but I do find it an interesting project.
Did you used the PCB's or just point to point?

Neither, I built mine onto vero-board and crammed it into a small Maplin case. With hindsight I might have preferred to use tag strips for the amp though, the VB worked very well for the PSU.

I am sure I have some pics somewhere if they would be useful to you.

G

[Don_Zalmrol]

The cap ordering problem is solved!

I'm going to build the 12AU7/6CG7 OTL amp and see what happens.
If it fails I can always learn to correct it or start again from scratch.

If you have them at hand on your computer, you may if you like.

Cheers,

Laurens

[Graham-R]

Originally Posted by Don_Zalmrol

The cap ordering problem is solved!

I'm going to build the 12AU7/6CG7 OTL amp and see what happens.
If it fails I can always learn to correct it or start again from scratch.

If you have them at hand on your computer, you may if you like.

Cheers,

Laurens

Best of luck with your build - the circuits for this type of amp are well proven so you should have little problem. The reason I did not use the Elektor amp board is that I felt the valves and electrolytic caps were placed too close together for longevity.

Hope this pic is of some help - I still have to complete the input switching and buffering.

Graham

[Don_Zalmrol]

That is quite a nice build you have there!

I Hope I will succeed to

Everything of the parts list is ordered and next week the parts will arrive!

[Graham-R]

This is probably a better picture for showing the layout. It was only meant as a prototype but it worked well enough to leave it as-is for at least the last 3 years. I shall rebuild the amp using tag-strip - but that won't be happenning for a long while yet.

Look forward to seeing some pics of your new toy !

[Don_Zalmrol]

Thx!

Will post them once all the parts arrive.
One parcel has arrived from Distrilect (Elfa) but is missing some (4) parts (backorder…).
And the one from JR is being assembled and will likely arrive next week!

[Craig Sawyers]

I haven't really been following this thread (even though I am a headphone junky).

But are you aware of HeadWize? http://gilmore2.chem.northwestern.edu/

There are a whole slew of valved headphone amp designs on it here http://gilmore2.chem.northwestern.ed...ndex.htm#tubes

Craig

[Craig Sawyers]

Originally Posted by murphyv310

Hi.
As an old timer with valves may I ask the silly question why we are feeding DC to heaters?
This is verging on audio foolery! .

I agree. I have three valved electrostatic headphone amps. A triode one based on 6S4's, and two groundbreaking designs by Kevin Gilmore. The ultimate is a replica (electrically) of the Stax SRM-T2, but with stabilised high voltage supplies. This uses 6DJ8 twin triodes as a balanced cascoded input stage, and EL34 output valves in push pull. But supporting them is a vast array of constant current and constant voltage sources using silicon. The other is a sort of version of the T2 (called the Blue Hawaii), but with solid state input circuits to drive the EL34's.

The point of all that description is this: All the heaters in all three amps are run from AC. And hum is completely inaudible. Even esoteric valve amps like those by Audio Research use AC heater supplies. Keep regulation for the high voltage supplies, because most valved amps are rather sensitive to hum on the anode and screen supplies.

Craig